1. Field of the Invention
The present invention relates to lens systems, such as a television lens and a video lens, that allow taking moving images.
2. Description of the Related Art
As to the lens systems that are used for a television lens and a video lens and allow taking moving images, there are many cases where a zoom lens unit conducting a zooming function and a focus lens unit conducting a focusing function are electrically driven using a motor in view of ease of operation for image taking and compatibility with a function to which an auto focus function is added.
In such a lens system, power supply to be consumed on a lens side is supplied from a camera, which the lens system is mounted on or incorporated with, to this lens system. In such a system, on the camera side, it is necessary to secure a capacity of power supply to be consumed on the lens side in addition to a capacity of power supply required to operate a function as a camera. However, demands for a small, lightweight and inexpensive camera impose a limitation on the capacity of power supply capable of being continuously supplied to the lens.
With this limitation, various proposals have been made to efficiently drive a motor of a lens.
For instance, Japanese Patent No. 3483211 discloses a lens for a television camera that identifies the type of a lens to be driven and changes the maximum value of current for driving the lens according to the type of the lens. Further, Japanese Patent Application Laid-Open No. 2000-111780 discloses a lens apparatus that changes the maximum value of drive current set with reference to a lens having a heavy weight to be driven according to the type of the lens apparatus.
A lens system that allows taking moving image and used for any of a television lens and a video lens has strongly been demanded to have higher magnification and higher image quality. In a case of supplying a new product, it is necessary to supply the market with products meeting these demands.
As a result of meeting these demands, there is a tendency of increasing both the weights of a zoom lens unit conducting a zooming function and a focus lens unit conducting a focusing function. In other hand, the capacity of power supply capable of being continuously supplied from the camera to the lens held by this camera is limited to the same capacity as the conventional level. Accordingly, in a case of simply increasing the maximum value of the drive current, the capacity of power supply from the camera is exceeded, which may cause failure of the camera.
For instance, in the conventional arts disclosed in Japanese Patent No. 3483211 and Japanese Patent Application Laid-Open No. 2000-111780, for the lens system having increased weights of both the zoom lens unit and the focus lens unit in comparison with a conventional lens units, the maximum value of current for driving the lens system according to the type of the lens units. However, if the maximum value of current for driving the lens system is not changed, drive characteristics become worse because of the increase in weight of the lens units in comparison with the conventional lens units.
If the reference drive current value is increased in accordance with the amount of increase in weight of the lens units, the increased current value exceeds the capacity of power supply on the camera side, which may cause failure of the camera.
Thus, it is an object of the present invention to provide a lens system that enables drive characteristics for the lens unit to be improved without affecting the camera.